Information-bearing discs and methods of fabrication

ABSTRACT

Information-bearing discs are fabricated from a first disc and a second disc. The first disc has a first substantially planar surface and a second surface opposite to the first surface. The second surface has first information recorded on it by means of local changes in the distance of the second surface from the first surface. The second disc is thicker than the first disc and has opposite, substantially planar, third and fourth surfaces. The fourth surface may have second information recorded on it by means of local changes in the distance of the fourth surface from the third surface. The second disc is secured to the first disc so that the fourth surface faces the second surface.

BACKGROUND OF THE INVENTION

This invention relates to information recording media in the form oflight-readable information-bearing discs, and more particularly to suchmedia which are made up of a front-side disc and a back-side discsecured to one another back to back to form a two-sided stack.

Light-readable information recording media in the form ofinformation-bearing discs are well known as shown, for example, byKramer U.S. Pat. No. 5,068,846. Commercially available compact discs("CDS") and compact disc read-only memories ("CD-ROMs") are examples ofrecording media of this general type.

More recently, digital versatile discs ("DVDs") have emerged whereinlight-readable information technology is employed to record movies andother similar real-time audio/visual programming (hereinafter "programinformation") on discs. To increase program information storagecapacity, information-bearing discs are formed from two discs (similarin appearance to conventional audio CDS) bonded back to back (see, forexample, Marquardt, U.S. Pat. No. 5,729,533, entitled "Two-sided,Light-Readable Information Recording Disc Stacks And Methods Of MakingSame," the entire contents of which are hereby incorporated byreference).

Relatively long audio/visual programs will not fit on one disc ofconventional audio CD size. As such, the "back-side" (i.e., the seconddisc bonded to the back of the "front-side" or first disc) of ainformation-bearing disc may often contain "spillover" audio/visualprogram information (i.e., information which would not fit on thefront-side of the information-bearing disc).

During the formation of a one-program-information-sideinformation-bearing disc (for example, a information-bearing discwherein only the front-side disc in the two disc stack contains programinformation), a substantially transparent plastic disc (i.e.,transparent enough to allow a light beam to remain detectable followinground trip travel through the plastic disc) with a substantially planarfirst surface (i.e., planar enough to allow a light source todifferentiate topographical features disposed on a surface of theplastic disc as the disc rotates) is provided. Opposite the firstsurface is a second surface having program information recorded on it bymeans of local changes in the distance of the second surface from thefirst surface. A light-reflecting coating (e.g., aluminum) is thendeposited (e.g., sputtered) over the second surface, thelight-reflecting coating conforming to the local changes of the secondsurface so that light is reflected back through the disc in accordancewith the program information recorded by means of the local changes.

Once the front-side program-information-bearing disc has beenfabricated, one-program-information-side information-bearing discformation is completed by bonding a back-sidenon-program-information-bearing disc (e.g., a disc without programinformation such as a blank disc or a disc with artwork, labels, orother ornamentation) to the front-side disc. This type ofnon-program-information-bearing disc does not normally contain alight-reflecting coating so that bonding may be performed using alight-curable bonding material which is cured by transmitting lightthrough the back-side plastic disc. Light can not be transmitted throughthe front-side disc because it contains a light-reflecting coating.

When a two-program-information-sides information-bearing disc (forexample, an information-bearing disc wherein both discs contain programinformation) is formed, a light-reflecting coating must be deposited onthe back-side disc so that program information recorded thereon may beread out. A light-reflecting coating may also be required if "pit art"(see previously incorporated Marquardt U.S. Pat. No. 5,729,533, entitled"Two-sided, Light-Readable Information Recording Disc Stacks And MethodsOf Making Same"), "enhanced pit art" (see, for example, Parette et al.U.S. patent application Ser. No. 08/714,302, entitled "Methods forProviding Artwork on Plastic Information Discs," filed Sep. 18, 1996,which is hereby incorporated by reference herein), or similar visibleimage information is contained on the back-side disc. (As used herein,the term "information," unmodified, refers to both program informationand visual image information such as pit art, enhanced pit art,holographic artwork, and other labels/ornamentation.)

Regardless of whether the information-bearing disc has information onone or both sides, it is important to the light-readability (i.e.,playability) of the information-bearing disc that the front-side discand back-side disc bond together to create a suitably flatinformation-bearing disc. Typically, when both front-side and back-sidediscs are formed by the same injection molding machine, both discs fittogether to create a suitably flat information-bearing disc. However,due to slight variations between molding machines, front-side andback-side discs formed on different molding machines may not bondtogether to create a suitably flat information-bearing disc, causingrippling and buckling (this warping is also referred to as the "potatochip" effect) which reduces the light-readability of theinformation-bearing disc.

A need therefore exists for a method of fabricating information-bearingdiscs such that the effects of variations in molding machines areminimized. Such a method will allow bonding together of front-side andback-side discs to create a substantially flat information-bearing discwith improved light-readability.

In view of the foregoing, it is an object of this invention to providesubstantially flat information-bearing discs and methods of fabrication.

SUMMARY OF THE INVENTION

These and other objects of the invention are accomplished in accordancewith the principles of the invention by providing a first disc and asecond disc. The first disc has a first substantially planar surface anda second surface opposite to the first surface. The second surface hasfirst information recorded on it by means of local changes in thedistance of the second surface from the first surface. A second discthicker than the first disc is provided. The second disc has opposite,substantially planar, third and fourth surfaces. The fourth surface mayhave second information recorded on it by means of local changes in thedistance of the fourth surface from the third surface. The second discis secured to the first disc so that the fourth surface faces the secondsurface.

During information-bearing disc formation, a first disc in theinformation-bearing disc is constructed generally as shown in KramerU.S. Pat. No. 5,068,846 entitled "Reflective Optical Record Carrier."This first disc therefore has light-readable program informationrecorded on it in the known fashion (e.g., local changes in distancebetween a first and a second surface of the disc covered with alight-reflecting coating so that light is reflected back through thedisc in accordance with the program information recorded by means of thelocal changes). The second disc in the information-bearing disc maysimilarly contain program information or some other (e.g., pit art,enhanced pit art, or holographic artwork).

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a conventionalone-program-information-side information-bearing disc.

FIG. 2 shows a cross-sectional view of a conventionaltwo-program-information-sides information-bearing disc.

FIG. 3 shows a cross-sectional view of a one-program-information-sideinformation-bearing disc made in accordance with this invention.

FIG. 4 shows a cross-sectional view of a two-program-information-sidesinformation-bearing disc made in accordance with this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to understand how the present invention improvesinformation-bearing disc formation, it is first necessary to understandhow conventional information-bearing discs are formed. With thisunderstanding, the problems with conventional information-bearing discfabrication techniques will be apparent.

FIG. 1 shows a conventional one-program-information-sideinformation-bearing disc 10 comprising a first disc 20 (the front-sidedisc) adhesively bonded to a second disc 30 (the back-side disc). FIG. 1illustrates the case in which only one of these two discs (first disc20) is needed for program information recording. Second disc 30 may beused for artwork, labeling, or left blank.

First disc 20 comprises a substantially smooth first surface 22a and asecond surface 22b having program information recorded on it by means oflocal changes 24 in the distance of the second surface 22b from thefirst surface 22a. A light-reflecting coating 26 is deposited oversecond surface 22b.

To form conventional one-program-information-side information-bearingdisc 10, second disc 30 is bonded to first disc 20. Second disc 30comprises substantially smooth third surface 32a and fourth surface 32band is bonded to first disc 20 with a light-curable bonding material 40disposed between second surface 22b and fourth surface 32b.

As shown in FIG. 1, first disc 20 and second disc 30 have thicknesses aand b, respectively, which are approximately the same. For example, fora DVD embodiment of information-bearing disc 10, typical values for aand b are approximately 0.6 mm. Thus, the ratio of thickness a/b isapproximately 1.

FIG. 2. shows a conventional two-program-information-sidesinformation-bearing disc 10' formed by bonding a first disc 20' (thefront-side disc) and a second disc 30' (the back-side disc) to oneanother back to back. As with first disc 20 of FIG. 1, first disc 20' ofFIG. 2 comprises a substantially smooth first surface 22a' and a secondsurface 22b' having program information recorded on it by means of localchanges 24' in the distance of the second surface 22b' from the firstsurface 22a'. A light-reflecting coating 26' is deposited over secondsurface 22b'.

To form conventional two-program-information-sides information-bearingdisc 10', second disc 30' is bonded to first disc 20', second disc 30'comprising substantially smooth third surface 32a' and fourth surface32b'. Unlike fourth surface 32b of FIG. 1, fourth surface 32b' of FIG. 2has program information recorded on it by means of local changes 34' inthe distance of fourth surface 32b' from third surface 32a' (as seconddisc 30' is a program-information-bearing disc). Additionally, alight-reflecting coating 36' is deposited over second surface 32b'.

As shown in FIG. 2, first disc 20' and second disc 30' have thicknessesc and d, respectively, which are approximately the same. For example,for a DVD embodiment of information-bearing disc 10, typical values forc and d are approximately 0.6 mm. Thus, the ratio of thickness c/d isapproximately 1.

As shown in FIG. 3, an illustrative one-information-sideinformation-bearing disc 10 made in accordance with this invention has afirst disc 20 (the front-side disc) and a second disc 30 (the back-sidedisc) bonded to one another back to back. First disc 20 and second disc30 have thicknesses a' and b', respectively, where b' is slightly largerthan a'. For example, for a DVD embodiment of information-bearing disc10, preferred values for a' and b' are approximately 0.6 mm and 0.7 mm,respectively. Thus, the ratio of thickness a'/b' is less than 1. Firstand second discs 20 and 30 of FIG. 3 are identical to first and seconddiscs 20 and 30 of FIG. 1 with the exception that the ratio of thicknessa'/b' for the embodiment of FIG. 3 is less than the ratio of thicknessa/b for the embodiment of FIG. 1.

As shown in FIG. 4, an illustrative two-information-sidesinformation-bearing disc 10" made in accordance with this invention hasa first disc 20" (the front-side disc) and a second disc 30" (theback-side disc) bonded to one another back to back. As shown in FIG. 4,first disc 20' and second disc 30' have thicknesses c' and d',respectively, where d' is slightly larger than c'. For example, for aDVD embodiment of information-bearing disc 10', preferred values for c'and d' are approximately 0.6 mm and 0.7 mm, respectively. Thus, theratio of thickness c'/d' is less than 1. First and second discs 20' and30' are identical to first and second discs 20' and 30' of FIG. 2 withthe exception that the ratio of thickness c'/d' for the embodiment ofFIG. 4 is less than the ratio of thickness c/d for the embodiment ofFIG. 2.

The added thickness of the second disc (the back-side disc) of theembodiments shown in FIGS. 3 and 4 substantially eliminates the ripplingand buckling problems often associated with bonding two discs formed ondifferent molding machines.

It will be understood that the foregoing is only illustrative of theprinciples of this invention, and that various modifications can be madeby those skilled in the art without departing from the scope and spiritof the invention. For example, the various dimensions and materialsmentioned herein are preferred, but other dimensions and materials canbe used if desired.

What is claimed is:
 1. An information recording medium comprising:afirst substantially transparent disc with a thickness of approximately0.6 mm, said first disc having a first substantially planar surface anda second surface opposite to said first surface, said second surfacehaving first information recorded on it by means of local changes in thedistance of said second surface from said first surface; and a seconddisc with a thickness of approximately 0.7 mm, said second disc havingopposite, substantially planar, third and fourth surfaces, said seconddisc being stacked on and secured to said first disc so that said fourthsurface faces said second surface.
 2. The recording medium defined inclaim 1 further comprising:a first light-reflecting coating on saidsecond surface for reflecting first light passed through said first discback through said first disc, said first coating conforming to saidlocal changes of said second surface so that said first light isreflected back through said first disc in accordance with the firstinformation recorded by means of said local changes, said first coatinghaving a first coating thickness which allows light-curable bondingmaterial curing through said first coating while maintaining sufficientfirst coating reflectivity for playability.
 3. The recording mediumdefined in claim 1 wherein said first information comprises programinformation.
 4. The recording medium defined in claim 1 wherein saidfirst information comprises pit art.
 5. The recording medium defined inclaim 1 wherein said first information comprises enhanced pit art. 6.The recording medium defined in claim 1 wherein said first informationcomprises holographic artwork.
 7. The recording medium defined in claim1 wherein said first light-reflecting coating comprises aluminum.
 8. Therecording medium defined in claim 7 further comprising a light-curablebonding material for securing said second disc to said first disc.
 9. Aninformation recording medium comprising:a first substantiallytransparent disc with a thickness of approximately 0.6 mm, said firstdisc having a first substantially planar surface and a second surfaceopposite to said first surface, said second surface having firstinformation recorded on it by means of local changes in the distance ofsaid second surface from said first surface; and a second disc with athickness of approximately 0.7 mm, said second disc having opposite,substantially planar, third and fourth surfaces, said fourth surfacehaving second information recorded on it by means of local changes inthe distance of said fourth surface from said third surface, said seconddisc being stacked on and secured to said first disc so that said fourthsurface faces said second surface.
 10. The recording medium defined inclaim 9 further comprising:a first light-reflecting coating on saidsecond surface for reflecting first light passed through said first discback through said first disc, said first coating conforming to saidlocal changes of said second surface so that said first light isreflected back through said first disc in accordance with the firstinformation recorded by means of said local changes, said first coatinghaving a first coating thickness which allows light-curable bondingmaterial curing through said first coating while maintaining sufficientfirst coating reflectivity for playability; and a secondlight-reflecting coating on said fourth surface for reflecting secondlight passed through said second disc back through said second disc,said second coating conforming to said local changes of said fourthsurface so that said second light is reflected back through said seconddisc in accordance with the second information recorded by means of saidlocal changes, said second disc being stacked on and secured to saidfirst disc so that said fourth surface faces said second surface. 11.The recording medium defined in claim 9 wherein said secondlight-reflecting coating has a second coating thickness which allowslight-curable bonding material curing through said second coating whilemaintaining sufficient second coating reflectivity for playability. 12.The recording medium defined in claim 9 wherein said first informationcomprises program information.
 13. The recording medium defined in claim9 wherein said first light-reflecting coating comprises aluminum. 14.The recording medium defined in claim 13 wherein said secondlight-reflecting coating comprises aluminum.
 15. The recording mediumdefined in claim 14 further comprising a light-curable bonding materialfor securing said second disc to said first disc.
 16. A method of makingan information recording medium, said method comprising:forming a firstsubstantially transparent disc with a thickness of approximately 0.6 mm,said first disc having a first substantially planar surface and a secondsurface opposite to said first surface, said second surface having firstinformation recorded on it by means of local changes in the distance ofsaid second surface from said first surface; forming a second disc witha thickness of approximately 0.7 mm, said second disc having opposite,substantially planar, third and fourth surfaces; and securing saidsecond disc to said first disc so that said fourth surface faces saidsecond surface.
 17. A method of making an information recording medium,said method comprising:forming a first substantially transparent discwith a thickness of approximately 0.6 mm, said first disc having a firstsubstantially planar surface and a second surface opposite to said firstsurface, said second surface having first information recorded on it bymeans of local changes in the distance of said second surface from saidfirst surface; forming a second disc with a thickness of approximately0.7 mm, said second disc having opposite, substantially planar, thirdand fourth surfaces, said fourth surface having second informationrecorded on it by means of local changes in the distance of said fourthsurface from said third surface; and securing said second disc to saidfirst disc so that said fourth surface faces said second surface.